Design of a 2-finger hand exoskeleton for finger stiffness measurements

Abstract

Recent studies of human arm movements have suggested that the control of stiffness may be important both for maintaining stability and for achieving differences in movement accuracy. Several studies in the robotic field demonstrated that grasp stiffness is useful for modelling and controlling manipulators but, even though it is accredited that having models of the human finger impedance would be very desirable for the control of anthropomorphous robot's hands, relatively few studies have focused on finger and hand stiffness. To allow the measurement of such entities at the finger level, an appropriate device capable of applying fast force transients while at the same time be able to monitor the finger movements is required. The work presented in this paper is a very detailed report about the design of a new hand exoskeleton system that will be used in our future works to investigate the finger stiffness range in different grasping postures and conditions.

Keywords:

• finger stiffness measurements
• impedance modelling
• dexterous manipulation
• physical human–robot interaction
• rehabilitation robotics

Acknowledgements

We thank Gabriel Baud-Bovy of S. Raffaele Medical Center, Milano, Italy, for a preliminary, really extensive, discussion of this research project and for his helpfulness during all the phases of the project. We also thank Prof. Thierry Pozzo, Prof. Darwin Caldwell, Marco Jacono and Lorenzo Masia of IIT, Genova, Italy, for their valuable advices on physiology, mechanics, computer-aided simulations and human impedance concepts.

Finally, we want to acknowledge the RobotCub consortium for having designed and maintained the development of the electronic devices included in this project.

Notes

¹The finger consisted of three joints, the distal interphalangeal (DIP), proximal interphalangeal (PIP) and metacarpophalangeal (MCP) joints (Kendall et al. 2005).

^aThe subject was given a visual feedback.

^bThe subject was not given a visual feedback.

²Futek LMD500 Medical Load Cell (Hand). Available from: http://www.futek.com/product.aspx?stock=FSH00125.

^aThe reported length is the length of the first phalanx while the inertia is the whole finger one.

³Maxon RE 25 Brushed Motors. Available from: http://shop.maxonmotor.com/ishop/article/article/118752.xml.

⁴Counts Per Turn – this parameter specifies the resolution of the encoder.

⁵Austriamicrosystems AS5045 12 Bit Programmable Magnetic Rotary Encoder. Available from http://www.austriamicrosystems.com.

⁶The control board and the magnetic encoder board have been developed as part of the project ‘RobotCub: European Commission Cognition Unit, Project no. IST-2004-004370’ (see Metta et al. 2008; RobotCub 2008).

⁷Controller-area network (CAN or CAN-bus) is a computer network protocol and bus standard officially released in 1986 at the SAE (Society of Automotive Engineers) congress and designed to allow micro-controllers and devices to communicate with each other without a host computer. It was designed specifically for automotive applications but is now also used in other areas.